The present invention relates to a radio transmission apparatus and, more particularly, to a radio transmission apparatus for transmitting a burst high-frequency signal which does not interfere with an adjacent channel in time-division multiplex communication.
A conventional radio transmission apparatus is constituted, e.g., by a logic processing unit 1, a modulation unit 2, and a high-frequency amplification unit 3, as shown in FIG. 10. The logic processing unit 1 includes a logic processing circuit 14 for performing differential conversion and the like, and a frequency divider 13 for frequency-dividing an input clock signal 12. The logic processing unit 1 receives input data 11 and the input clock signal 12 and outputs a differential conversion signal. Note that the input clock signal 12 is frequency-divided up to the speed of a data rate by the frequency divider 13. Assume, in this case, that the frequency division ratio is 1/8. The logic processing circuit 14 receives the input data 11 and a clock 17 obtained by frequency division and extracts a timing signal for each time slot when time-division transmission is performed. Note that this timing signal may be externally supplied. The timing signal is supplied, as a burst timing signal 15, to the high-frequency amplification unit 3 and the modulation unit 2 on the subsequent stage of the logic processing unit 1. The modulation unit 2 includes a digital filter 21 and a modulation circuit 22. The digital filter 21 receives a data signal 16 processed by the logic processing circuit 14 and performs waveform shaping of a transmission spectrum. For example, the digital filter 21 has a Gaussian filter function and serves to perform waveform shaping, e.g., conversion of an input data signal into a modulation signal for MSK (minimum shift keying). The burst timing signal 15 serves to reset the digital filter 21 at the start of each time slot so as to synchronize data in each time slot with a clock. The modulation circuit 22 modulates a carrier using an output signal from the digital filter 21 as a modulation signal. The high-frequency amplification unit 3 amplifies a modulated carrier 23 output from the modulation unit 2. In addition, in transmitting a signal for each time slot as a burst-like signal, the high-frequency amplification unit 3 performs waveform shaping to smooth the leading and trailing edges of the burst-like signal to prevent unnecessary spread of a spectrum when the signal rises and falls. The burst timing signal 15 is used for synchronization with this waveform shaping operation.
The waveform shaping operation will be described next with reference to FIGS. 11A to 11C. When the burst timing signal 15 is input with respect to the burst-like data signal 23 shown in FIG. 11A at the timing shown in FIG. 11C, a high-frequency output signal 24 is waveform-shaped to have smooth leading and trailing edges, as shown in FIG. 11B, thereby preventing unnecessary spread of a spectrum caused by abrupt changes in amplitudes at the leading and trailing edges of a transmission wave.
In this conventional radio transmission apparatus, a burst timing signal serves to reset not only the high-frequency amplification unit 3 but also the digital filter 21. If the digital filter 21 is reset, an output from the digital filter 21 exhibits discontinuity or an abrupt change at the time of the reset operation. As shown in FIG. 11C, when a reset point, i.e., the burst timing signal 15, is located before the leading edge of the high-frequency output signal 24, even if a discontinuous point appears at that time point and a spectrum spreads, the signal is not radiated from an antenna 4. Even if the signal is radiated, since the signal level is very low, an adjacent frequency channel is not influenced.
In a base station of an automobile telephone system, adjacent time slots may be used at once. This state is shown in FIGS. 12A to 12C. More specifically, as shown in FIG. 12A, since time slots A and B of the burst-like data signal 23 are continuously transmitted, the digital filter 21 is reset by the burst timing signal 15 at the boundary between the time slots A and B of the burst-like data signal 23 before the level of the high-frequency output signal 24 sufficiently decreases, as shown in FIG. 12C. The high-frequency signal 24 shown in FIG. 12B, whose spectrum is not smooth and unnecessary spreads owing to data discontinuity caused by this reset operation, is radiated from the antenna 4. For this reason, the radiated signal interferes with an adjacent frequency channel to make it difficult to perform proper communication.